By "roasted zinc sulphide concentrates" is meant the product obtained by roasting a zinc sulphide bearing concentrate. The term zinc calcine is also used to refer to such products. Typically, roasted zinc sulphide concentrates contain zinc oxide and other components, including zinc sulphate, zinc sulphide, mixtures of metal oxides such as copper, lead, calcium, cadmium, and magnesium, zinc ferrite, magnetite, possibly some haemetite, and usually some metal silicates, sulphate and sulphides.
Zinc sulphide bearing concentrates are commonly roasted in a fluid bed roaster. In the fluid bed roasting of zinc concentrates, the concentrates are continuously fed into the roaster chamber, wherein air blown up through the chamber converts sulphides to oxides releasing sulphur into the gas stream chiefly as sulphur dioxide, with the development of heat. Smaller and lighter particles in the feed concentrate tend to be carried out of the chamber with the gas stream and are collected by downstream equipment. Larger and heavier particles tend to remain in the roaster fluidized bed through which the moving air is blown. Since the bed is full during continuous operations, a bed overflow stream outlet is provided by which further accumulation of coarse solids is prevented by allowing bed material to overflow out of the chamber. This material is referred to as "bed overflow calcine". In some zinc refineries, calcine is composed of an approximately 50:50 by weight mixture of bed overflow calcine and fine material that is recovered from the gas stream, as described above. This mixture is then used as a feed material in the production of zinc metal and is referred to as normal calcine at least in some cases, such as at the zinc refining from which samples were used in the U.S. Pat. No. 5,028,410 hereafter referred to below.
In accordance with a previous invention, U.S. Pat. No. 5,028,410 aqueous ammoniacal ammonium carbonate solutions are used to selectively leach zinc from roasted zinc sulphide concentrates, leaving a substantial portion of the iron components in the leach residue.
Ammoniacal ammonium carbonate solutions have been suggested for treating zinc scrap and mini-steel plant baghouse dusts to recover zinc oxide. However, it was not previously recognized that such solutions could be used to leach roasted zinc sulphide concentrates, which are dissimilar to zinc scrap and baghouse dust, to produce such high quality, high surface area zinc oxide as are obtained by U.S. Pat. No. 5,028,410. Zinc oxide with the high specific surface area produced by U.S. Pat. No. 5,028,410 can be classed as premium zinc oxide.
A new discovery now reveals that by the use of bed overflow calcine alone, rather than normal calcine as discussed above, a number of improvements result. This is the essence of the present invention.
It is to be noted here that the ratio of bed overflow calcine to dust carryover calcine can vary widely at different zinc refineries. The ratio will not always be approximately 50:50. This would, in no way, detract from the practice of the present invention.